The present invention relates to a process for producing a graft-copolymer for blending which is capable of imparting a high impact resistance to a thermoplastic resin having a relatively low impact resistance without impairing its transparency and luster or gloss when it is blended with the thermoplastic resin. Further, the present invention aims at producing a coagulated rubber latex of diene type which has a high degree of coagulation and a relatively narrow range of particle size distribution as a base material for a graft-copolymer advantageously on an industrial scale without the formation of rubber masses or lumps and macroparticles.
Heretofore, a latex containing coagulated and enlarged rubber particles which are formed by adding an electrolyte as a coagulating agent to a diene type rubber latex has been used as a base material for a graft-copolymer. That is, a rubber polymer constituting the rubber particles is used as a trunk polymer of a graft-copolymer.
For example, in the case where a monomer selected from styrene, acrylonitrile, methyl methacrylate and the like is added to a latex of a conjugated diene rubber of a butadiene polymer or butadiene-styrene copolymer which has been obtained through polymerization with the use of a fatty acid soap and is subjected to graft-polymerization, thereby producing an impact resistance improving agent for a thermoplastic resin such as a graft-copolymer of an acrylonitrile-butadiene-styrene resin (hereinafter referred to as ABS resin, for brevity) or a methyl methacrylate-butadiene-styrene resin (hereinafter referred to as MBS resin, for brevity), there has been known a process wherein an acid is previously added to a rubber latex to reduce the pH thereof and to coagulate and enlarge the rubber particles thereby to provide a relatively large particle size, and thereafter, the resultant rubber latex is subjected to graft-polymerization to produce a resin having a high impact resistance. There has been known also a process wherein a salt is added to rubber latex to coagulate and enlarge the rubber particles, and the resultant latex is then graft-polymerized to obtain a resin having improved impact resistance. These processes are disclosed in Japanese Patent Publication Nos. 3112/1967, 22541/1967 and 3015/1968. The resultant graft-copolymers are used as impact resistance improving agents, for example, for vinyl chloride resins, styrene-methyl methacrylate copolymers, and styrene-acrylonitrile copolymers.
However, when the above mentioned processes for coagulating and enlarging rubber particles were used, it was difficult to produce easily on an industrial scale a latex containing rubber particles of a high degree of coagulation and a relatively narrow range of particle size distribution. That is, the process in which an acid is added to coagulate the rubber particles in a rubber latex is disadvantageous in that when a quantity of the acid required to amply coagulate the rubber particles is added, the latex, in general, becomes acidic, and the dispersion stability of the latex is reduced. When the concentration of the latex is high, there is a tendency for macroparticles and rubber masses or lumps to develop.
On the other hand, the process wherein a salt is added requires a large quantity of salt and reduces the stability of the latex, as a result of which rubber masses tend to be formed. In addition, because these processes result in a broad particle size distribution and cause the formation of macroparticles when the resultant graft-copolymers as an impact resistance improving agent are added to, for example, vinyl chloride resins and the resulting composition is shaped, the shaped article is remarkably inferior in transparency and surface gloss to shaped articles not containing such improving agents.